The optical phenomenon, reflection, is a product of the change in medium in which the light is travelling. The medium is characterized optically by its refractive index (n), which quantifies the speed of light in that medium with respect to a vacuum. The mediums, of which most systems are concerning, are air and glass and have n values of ca. 1 and 1.5 respectively. The fraction of light being reflected as the light passes between two mediums can be calculated using the Fresnel equation:
                    r        =                              (                                                            n                  o                                -                                  n                  s                                                                              n                  o                                +                                  n                  s                                                      )                    2                                    [                  MATH          .                                          ⁢          1                ]            
where no is the refractive index (RI) of the first medium (air) and ns is the RI of the second medium (glass). The main strategies for reducing reflectance are to either; reduce the difference between the RI of the glass and air mediums by addition of a material with a RI value somewhere in between them (optimum RI 1.22); or to employ a coating with thickness λ/4 for a specific wavelength resulting in deconstructive interference of the reflected radiation. The most common antireflective coating (ARC), which is made from MgF2, usually employs both strategies. It has an RI of 1.38, which although is higher than the optimum value, no other known material combines a lower RI with the same degree of durability. Subsequently, by controlling the thickness of the coating to be λ/4, the MgF2 can provide sufficient anti-reflectance over a broad range of wavelengths relative to λ.
Effective ARCs can be categorised into two forms; homogeneous and inhomogeneous. The two types can exist as single layer multi-stacks as well as patterned subwavelength structures (SWS). Single layer homogenous forms are limited in their ability to reduce reflectance and must have a controlled thickness of λ/4 to become a practical option. Single layer inhomogeneous forms, which possess a gradient refractive index (GRIN) are most effective in suppressing the Fresnel reflections at an interface. They impart anti-reflective property to an interface not only over a broad spectral range but also over wider angles of incidence. Conventionally, a gradient refractive index (GRIN) has been achieved by depositing multiple layers of materials of successively reducing refractive index. However, multi-layer ARCs often deteriorate over time because of the debonding caused by thermal mismatch and poor interfacial adhesion. Incompatible mechanical and thermal properties of polymer antireflective surfaces and their durability pose major operational challenges for applications of GRIN.
There is therefore a need to provide a coating material and antireflective material comprising a substrate coated with the coating material that overcomes or at least ameliorates, one or more of the disadvantages described above.